In the field of hydrogen production, MoS2 demonstrates good catalytic properties for the hydrogen evolution reaction (HER) which improve when doped with metal cations. However, while the role of sulfur atoms as active sites in the HER is largely reported, the role of metal atoms (i.e. molybdenum or the dopant cations) has yet to be studied in depth. To understand the role of the metal dopant, we study MoS2 thin films doped with Co and Mn ions. We identify the contribution of the electronic bands of the Mn and Co dopants to the integral valence band of the material using in situ resonant photoemission measurements. We demonstrate that Mn and Co dopants act differently: Mn doping favors the shift of the S-Mo hybridized band towards the Fermi level, while in the case of Co doping it is the less hybridized Co band that shifts closer to the Fermi level. Doping with Mn increases the effectiveness of S as the active site, thus improving the HER, while doping with Co introduces the metallic site of Co as the active site, which is less effective in improving HER properties. We therefore clarify the role of the dopant cation in the electronic structure determining the active site for hydrogen adsorption/desorption. Our results pave the way for the design of efficient materials for hydrogen production via the doping route, which can be extended to different catalytic reactions in the field of energy applications.

The integration of lead sulfide quantum dots (QDs) with a high-conductivity material that is compatible with a scalable fabrication is an important route for the applications of QD-based photodetectors. Herein, we first developed a broadband photodetector by combining amorphous ZnO and PbS QDs, forming a heterojunction structure. The photodetector showed detectivity up to 7.9 × 1012 and 4.1 × 1011 jones under 640 and 1310 nm illumination, respectively. The role of the oxygen background pressure in the electronic structure of ZnO films grown by pulsed laser deposition was systematically studied, and it was found to play an important role in the conductivity associated with the variation of the oxygen vacancy concentration. By increasing the oxygen vacancy concentration, the electron mobility of amorphous ZnO layers dramatically increased and the work function decreased, which were beneficial for the photocurrent enhancement of ZnO/PbS QD photodetectors. Our results provide a simple and highly scalable approach to develop broadband photodetectors with high performance.

This article reports on the interplay between structural properties of the interface region and emerging proton conductivity in thin films of insulating BaZrO3 perovskite deposited by pulsed laser ablation onto NdGaO3 wide-band-gap insulators. High-resolution transmission electron microscopy and surface x-ray diffraction reveal the presence of a large number of misfit dislocations at the interface, allowing the full relaxation of the epitaxial strain. An analysis of the x-ray diffraction patterns reveals strain relaxation that occurs over a thickness of about 3 nm, equally divided between the film and the substrate. Electrical impedance spectroscopy measurements show a sizeable proton conductance that can mainly be attributed to the defective interface. Assuming that proton conduction occurs in the interface layer, values of interface conductivity of about 0.5 S/cm at 650 degrees C are estimated with an activation energy of about 0.86 eV. Experimental findings are explained assuming that the defective interface layer can accommodate a very large concentration of carriers whose mobility is somewhat hindered by the high concentration of structural defects at the interface.

The interface between mismatched perovskite oxides presents novel physical and chemical features of interest for the development of engineered materials with tailored properties. In this article, we report on the presence and properties of a regular network of misfit dislocations (MDs), which self‐assembles at the interface between a thin film of the proton conductor BaZr0.8Y0.2O2.9 and the substrate, the (110) oriented NdGaO3 wide‐gap insulator. The conductivity properties of this system strongly depend on the atmosphere at which a thermal annealing is carried out. Namely, a conductivity increase is observed after annealing at 450 °C in wet atmosphere. An annealing in dry environment at the same temperature brings back the conductivity to its pristine value. X‐ray diffraction measurements show a strong increase of the strain field associated with the dislocation network, after annealing in wet atmosphere, suggesting that hydroxyl groups mostly accumulate in the cores of the interface MDs rather than in the bulk of the BaZr0.8Y0.2O2.9 thin film. A further annealing in dry environment allows to recover the initial value of the strain field. The reported data indicate a strong involvement of the interface MDs in the transport properties of perovskite oxide interfaces.

This paper reports on the room temperature ambipolar diffusion of oxygen vacancies and electrons in epitaxial films of pure CeO2 and 20% Sm-doped CeO2 a few tens of nanometers thick. To this aim, we fabricated heteroepitaxial structures consisting of an epitaxial ceria film sandwiched between a Pt upper electrode and a conducting substrate of Nb-doped SrTiO3 (lower electrode). The observed decay of the electrical conductivity was investigated. In particular, following this approach, we could measure room temperature diffusion coefficient in thin monocrystalline dielectric layers. While pure ceria is interesting for memory device applications, in doped ceria the resistive switching involves diffusion mechanisms that occur on much longer time scales. Under this respect, doped ceria is particularly suitable to be used in artificial neuronal devices.

The role of trivalent rare-earth dopants on the cerium oxidation state has been systematically studied by in situ photoemission spectroscopy with synchrotron radiation for 10 mol % rare-earth doped epitaxial ceria films. It was found that dopant rare-earths with ionic radius foster the fotthation of Ce3+ by releasing the stress strength induced by the cation substitution. With a decrease of the dopant ionic radius from La3+ to Yb3+, the out-of-plane axis parameter Of the crystal lattice, decreases without introducing Macroscopic defects. The high crystal quality of Our firms allowed us to Comparatively study both the ionic conductivity and surface reactivity ruling out the influence of structural defects. The measured increase in the activation energy of films and their enhanced surface reactivity can be explained in terms of the dopant ionic radius effects on the CO4+ -> Ce3+ reduction as a result of lattice relaxation. Such findings open new perspectives in designing ceria-based materials with tailored properties by choosing suitable cation substitution.

Samaria-doped ceria (SDC) thin films are particularly important for energy and electronic applications such as microsolid oxide fuel cells, electrolyzers, sensors, and memristors. In this paper, we report a comparative study investigating ionic conductivity and surface reactions for well-grown epitaxial SDC films varying the samaria doping concentration. With increasing doping above 20 mol % of samaria, an enhancement in the defect association is observed by Raman spectroscopy. The role of such associated defects on the films̀ oxygen ion transport and exchange is investigated by electrochemical impedance spectroscopy and electrochemical strain microscopy (ESM). The measurements reveal that the ionic transport has a sharp maximum in ionic conductivity and drops in its activation energy down to 0.6 eV for 20 mol % doping. Increasing the doping concentration further up to 40 mol %, it raises the activation energy substantially by a factor of 2. We ascribe the sluggish transport kinetics to the "bulk" ionic-near ordering in case of the heavily doped epitaxial films. Analysis of the ESM first-order reversal curve measurements indicates that these associated defects may have a beneficial role by lowering the activation of the oxygen exchange "surface" reaction for heavily doped 40 mol % of samaria. In a model experiment, through a solid solution series of samaria doped ceria epitaxial films, we reveal that the occurrence of associated defects in the bulk affects the surface charging state of the SDC films to increase the exchange rates. The implication of these findings is the design of coatings with tuned oxygen surface exchange by controlling the bulk associated clusters for future electrocatalytic applications.

We report on the characterization of resistive switching devices based on epitaxial CeO2 thin films as a functional material. CeO2 epitaxial thin films were grown by the pulsed laser deposition technique on conductive substrates. Platinum and titanium nitride top electrodes (TE) were successively deposited. Very good performances, in terms of resistivity switching and multilevel operation capability, were obtained using the Pt TE. The dependence of the low resistance and high resistance state on the TE material and on the CeO2 film thickness were explained. The electrical characteristics of these heterostructures make them promising as synapse for neuromorphic computation, but suggest also their use with multi-valued digital systems or multibit memory cells.

Manganese spinel ferrite nanoparticles were synthesized by a solvothermal route based on high temperature decomposition of metal nitrates in the presence of different contents of Triethylene glycol. This simple and low cost method can be applied to prepare large quantities of nanoparticles (tens of grams). Powder X-ray diffraction (PXRD) and Transmission Electron Microscopy (TEM) confirmed that nanoparticles with a good crystalline quality were obtained. A good agreement between the average particle size calculated by PXRD and TEM was observed. Fourier-transform infrared spectra showed that polymer molecules have the tendency to form bonds with the surface of ferrite nanoparticles reducing the surface spin disorder, and then enhancing the saturation magnetization (MS). Therefore, much higher MS value (up to ∼91emu/g at 5K) was observed compared with that of bare nanoparticles without surfactant. The blocking temperature showed a remarkable shift to lower values with increasing the polymer starting amount. In addition, by increasing the polymer initial content, a more homogeneous size distribution was obtained and the initial strongly interacting superspin glass behavior changed to a weakly interacting superparamagnetic state.

In this paper, we report on BaZr0.8Y0.2O3-x (BZY) thin films grown on highly mismatched NdGaO3 (110) substrates by RHEED assisted pulsed laser deposition. The conduction and electrochemical performances are studied by Electrochemical Impedance Spectroscopy and Electrochemical Strain Microscopy respectively. Conductivity, as well as electrochemical response improves while decreasing the thickness, indicating that the proton movement is prompt at the defective interface region. Structural defects at the interface between film and substrate are clearly displayed by x-ray diffraction, RHEED patterns and transmission electron microscopy. The role of chemical defects on BZY film properties is elucidated by Hard X-ray Photoelectron Spectroscopy. Our results demonstrate that both structural dislocations and chemical defects influence the proton conduction and reaction process in BZY thin films.

We have grown nanostructured films of Zn/Al Layered Double Hydroxide (LDH) on different substrates by combining the deposition of an aluminum micropatterned thin layer with a successive one-step room-temperature wet-chemistry process. The resulting LDH film is made of lamellar-like nanoplatelets mainly oriented perpendicular to the substrate. Since the aluminum layer acts as both reactant and seed for the synthesis of the LDH, the growth can be easily confined with submicrometric-level resolution (about ±0.5 μm) by prepatterning the aluminum layer with conventional photolithographic techniques. Moreover, we demonstrate real-time monitoring of the LDH growth process by simply measuring the resistance of the residual aluminum film. If the aluminum layer is thinner than 250 nm, the morphology of LDH nanoplatelets is less regular and their final thickness linearly depends on the initial amount of aluminum. This peculiarity allows accurately controlling the LDH nanoplatelet thickness (with uncertainty of about ±10%) by varying the thickness of the predeposited aluminum film. Since the proposed growth procedure is fully compatible with MEMS/CMOS technology, our results may be useful for the fabrication of micro-/nanodevices.

Yttrium-doped barium zirconate (BZY) thin films recently showed surprising electric transport properties. Experimental investigations conducted mainly by electrochemical impedance spectroscopy suggested that a consistent part of this BZY conductivity is of protonic nature. These results have stimulated further investigations by local unconventional techniques. Here, we use electrochemical strain microscopy (ESM) to detect electrochemical activity in BZY films with nanoscale resolution. ESM in a novel cross-sectional measuring setup allows the direct visualization of the interfacial activity. The local electrochemical investigation is compared with the structural studies performed by state of art scanning transmission electron microscopy (STEM). The ESM and STEM results show a clear correlation between the conductivity and the interface structural defects. We propose a physical model based on a misfit dislocation network that introduces a novel 2D transport phenomenon, whose fingerprint is the low activation energy measured.

Magnetic properties of iron oxide nanoparticles with spinel structure are strictly related to a complex interplay between cationic distribution and the presence of a non-collinear spin structure (spin canting). With the aim to gain better insight into the effect of the magnetic structure on magnetic properties, in this paper we investigated a family of small crystalline ferrite nanoparticles of the formula CoxNi1-xFe2O4 (0 ≤x≤ 1) having equal size (≈4.5 nm) and spherical-like shape. The field dependence of magnetization at low temperatures indicated a clear increase of magnetocrystalline anisotropy and saturation magnetization (higher than the bulk value for CoFe2O4: ∼130 A m(2) kg(-1)) with the increase of cobalt content. The magnetic structure of nanoparticles has been investigated by Mössbauer spectroscopy under an intense magnetic field (8 T) at a low temperature (10 K). The magnetic properties have been explained in terms of an evolution of the magnetic structure with the increase of cobalt content. In addition a direct correlation between cationic distribution and spin canting has been proposed, explaining the presence of a noncollinear spin structure in terms of superexchange interaction energy produced by the average cationic distribution and vacancies in the spinel structure.

Scanning probe bias techniques have been used as a method to locally dope thin epitaxial films of La2CuO4 (LCO) fabricated by pulsed laser deposition. The local electrochemical oxidation of LCO very efficiently introduces interstitial oxygen defects in the thin film. Details on the influence of the tip voltage bias and environmental conditions on the surface morphology have been investigated. The results show that a local uptake of oxygen occurs in the oxidized films.

Water adsorption, splitting, and proton liberation were investigated on Sm0.1Ce0.9O2-δ thin films by scanning probe microscopy. An irreversible volume expansion was observed by applying a positive bias with increased temperature. The volume expansion is also linearly dependent on the relative humidity. A reversible water adsorption process and its effect on the conductivity were also investigated by electrochemical strain microscopy and first order reversal curve under a number of experiment conditions. The presence of a Ce3+ along with OH groups, detected by hard x-ray photoemission spectroscopy established a clear correlation between the water incorporation and the valence state of Ce.

A systematic study by reversible and hysteretic electrochemical strain microscopy (ESM) in samples of cerium oxide with different Sm content and in several working conditions allows disclosing the microscopic mechanism underlying the difference in electrical conduction mechanism and related surface activity, such as water adsorption and dissociation with subsequent proton liberation. We have measured the behavior of the reversible hysteresis loops by changing temperature and humidity, both in standard ESM configuration and using the first-order reversal curve method. The measurements have been performed in much smaller temperature ranges with respect to alternative measuring techniques. Complementing our study with hard X-ray photoemission spectroscopy and irreversible scanning probe measurements, we find that water incorporation is favored until the doping with Sm is too high to allow the presence of Ce3+. The influence of doping on the surface reactivity clearly emerges from all of our experimental results. We find that at lower Sm concentration, proton conduction is prevalent, featured by lower activation energy and higher electrical conductivity. Defect concentrations determine the type of the prevalent charge carrier in a doping dependent manner.

We present the results of our project for the design and construction and on-sky test of silicon grisms. The fabrication of such devices is a complex and critical process involving litho-masking, anisotropic etching and direct bonding techniques. After the successful fabrication of the silicon grating, we have optimized the bonding of the grating onto the hypotenuse of a silicon prism to get the final prototype. After some critical phases during the experimentation a silicon grism with 363 grooves/mm and a blaze angle of 14 degrees has been eventually fabricated. The application of an A/R coating on both the surfaces has been the last step: this procedure is critical because of the groove geometry of the diffraction grating, whose performace might be compromised by the coating. Then, the grism was inserted in the filter wheel of the Near Infrared camera NICS, at the focal plane of the National Galileo Telescope (TNG), the 3.5 m Italian facility in the Canary Islands (E). The result of the on-sky tests are given in detail.

A study of the structure and transport properties of highly textured, epitaxial oriented BaZr0.8Y0.2O3−x thin films grown on NdGaO3(110) is reported. Films have been grown by pulsed laser deposition and their conductivity studied as a function of temperature and thickness. The results show an increased conductance as the sample thickness decreases. The measured conductivity corresponding to an in-plane conductivity of 20 S cm−1 has been systematically observed in the range of 550–600 °C for several 10 nm-thick films. The high values of conductivity are possibly related to the high densities of defects, mostly dislocations at the interface of the film with the substrate.

The effects originating from the proximity between the ferromagnetic and the antiferromagnetic phase of a CoPt/NiO bilayer, grown at 670K by Pulsed Laser Deposition, have been investigated from the point of view of the chemical properties (through Hard X-ray Photoelectron Spectroscopy, HAXPES) and of the magnetic behavior (by measuring hysteresis loops in the temperature range 5–300K both after cooling in zero external field and after cooling from T=380K in a field of 1T). At T=5K, the coercivity, measured after zero-field-cooling, is ~168mT, to be compared to that of a reference CoPt layer of ~87mT. Such magnetic hardening of the ferromagnetic CoPt phase is ascribed to the magnetic exchange interaction at the interface with the antiferromagnetic NiO phase, which is also responsible for the horizontal shift of the loop, observed only after field-cooling (exchange bias effect). Actually, the latter effect persists up to room temperature (exchange fields μ0Hex ~60mT and ~8mT were observed at T=5 and 300K, respectively). Hence, it can be deduced that the CoPt and NiO phases are efficiently coupled by the exchange interaction, despite the chemical inhomogeneity observed at the interface region. In fact, the HAXPES analysis reveals that a chemical reduction of the NiO phase takes place in the interface region, resulting in the formation of metallic Ni. On the other hand, this inhomogeneity of the interface is proposed to be at the origin of the peculiar shape of the field-cooled loop at T=5K, featuring a double reversal of the magnetization.

After depositing on glass substrates aluminum thin film micro-patterned tracks, we have used a wet chemistry process in order to transform the aluminum metal into Zn/Al layered double hydroxides (LDHs). This technique consists in placing the substrates into a zinc nitrate solution with a basic agent at temperatures lower than 90 °C. Such a hydrothermal method is very simple, low cost, CMOS/MEMS compatible, suitable for large area substrates and results in the growth of Zn/Al LDHs thin films with nanoplatelets shape. We have then tested the nanostructured Zn/Al LDH micro-sized tracks as a humidity sensor and found a high sensitivity at room temperature. Our approach seems ideal for the fabrication of CMOS-compatible, low power, on-chip integrated humidity micro-sensors.